Its much like pumps and loop restriction if you are familiar with that.

Any radiator ( or any air obstruction .. filter, fan grill) has a certain restriction to airflow. The denser the FPI and the thicker the inner tubes the higher it is (roughly)

This is equivalent to the pressure drop graphs you see for water blocks etc ... the higher the flow, the more resistance.

Fans have a PQ curve that will trace how much pressure they exert on the air from 0 flow rate all the way up to their maximum flow rate that is only seen with 0 restriction.

Fans marketed as "high static pressure" will usually have a curve that starts high on the left side, indicating high pressure at 0 flow and will dive down quite steeply to their max flow rate and 0 pressure.

Fans with very steep blades will start off relatively low on the pressure at 0 flow but the graph will decrease at a much more shallow rate to end up at a higher flow rate at 0 pressure.

This graph is from the Noiseblocker eloop datasheet.

The yellow curve is very roughly the same yellow curve from the graph above for the Black Ice rad in converted units

The Black dots are roughly the curve for the Scythe GT AP15 from above converted in to the units for this graph.

The straight red line is an imaginary High static pressure fan

The straigh blue line is an imaginary high air flow / low static pressure fan

We can see that the AP 15 and eloop B12-3 (green curve) cross the line of the rad curve at very close to the same point, even though the eloop is rated to some 20 cubic meters an hour higher flow. At full speed both fans would produce about 60 m3/h air flow rate.

The eloop B12-ps (dashed blue line) is rated to the same max flow rate as the AP15 but the higher static pressure of the AP15 means it crosses the rad curve at a much higher flow rate. 50 vs 65ish m3/h air flow.

The imaginary fan lines are just to show how it is possible for fans with very different manufacturer specs to end up being relatively close to the same flow rate through a radiator or grill.

A more restrictive rad will have a steeper restriction curve that will favour fans that stick to the high pressure parts of graphs. Lower restriction rads have a more gently sloping restriction curve and are a little more tolerant of different fan types as their curves will intersect at flow rates that are not too far apart.

Obviously this isn't meant to be a scientific comparison of different fans on rads ... just to outline the concept and how it works, so go easy.

The only reason these two fans were used is that they are both made by the very few companys that actually publish proper data.

The radiator air restriction graph is a very old one too so don't think that it applies exactly to modern rads